1. Field of the Invention
This invention relates to an improved chemical milling solution and method for milling, etching or pickling metal products therewith. More particularly, the invention relates to a bath composition and method for milling or pickling titanium workpieces, such as forgings or the like.
2. Technology Review
As used herein, the term "milling" shall mean the selective and controlled removal (or corrosion) of metal (or metal oxides) from a part or object by chemical milling, etching and/or pickling. Most milling procedures form metal product of a desired thickness and/or configuration by removing metal from treated workpieces and imparting greater weight savings to aerospace parts or the like. Milling operations are typically performed after particular metal parts have been formed by casting, forging, extrusion or rolling; and heat treated. Milling is also used to make shapes which cannot otherwise be machined by conventional chipmaking techniques, or which can only be machined by known methods at unreasonably high cost. For many parts, masking of certain areas is done to prevent their exposure to a corrosive milling solution.
As used for the description of this invention, "milling" shall also include metal etching, the controlled removal of metal for dimensional and shape control, and metal cleaning or pickling, i.e., the removal of embrittled oxidized surfaces. For titanium alloys, embrittled surfaces are sometimes referred to as alpha-case. Such surfaces typically result from elevated temperature exposure in the manufacturing process, i.e., casting, rolling, extrusion, forging or the like.
Any chemically dissolvable metal may be subjected to treatment by the aforementioned milling practices. Alloys of aluminum, beryllium, magnesium, titanium and various steels are the most commonly milled metal products. Refractory metals such as molybdenum, tungsten, niobium (columbium) and zirconium may also be chemically etched in the same manner. The workpieces treated by milling (i.e. chemical, etching and/or pickling) need not be limited by size provided a large enough bath of milling solution can be maintained. Milled parts may be cast, forged, extruded or rolled. Their end shapes may be flat, tubular or in any of the complex configurations required by today's manufacturers of aerospace and other parts.
The first chemical milling practices are believed to have occurred around 2500 B.C., when ancient Egyptians used citric acid to etch copper jewelry. Current industrial milling U.S. Pat. No. 2,739,047. Numerous evolutions to milling patented over 35 years ago. Many of these solution developments depended on the particular metal alloy being milled.
For titanium and titanium-based alloys, Chen U.S. Pat. No. 4,900,398 claims a milling method which uses an aqueous solution consisting essentially of 1-5% hydrofluoric acid, about 1.5-4% chlorate ion and, optionally, up to about 20% of an acid selected from the group consisting of H.sub.2 SO.sub.4, HCl and HNO.sub.3.
Kremer et al. U.S. Pat. No. 4,314,876 discloses a milling solution consisting essentially of: 3-10 wt. % ammonium bifluoride; 5-15 wt. % nitric acid, or its equivalent as ammonium nitrate, sodium nitrate or potassium nitrate; 2-25 wt. % hydrochloric acid when ammonium nitrate, sodium nitrate or potassium nitrate is used as the nitrate source; up to 1 wt. % wetting agent; and 92-49 wt. % water. According to the examples, this solution removes Ti metal at rates ranging from 0.000027 to 0.00074 mils/side/minute.
In Coggins et al. U.S. Pat. No. 4,116,755, there is claimed a method and composition for milling titanium without excessive hydrogen absorption. The composition comprises, per liter of solution: about 126-700 grams of pure nitric acid, or its equivalent; the equivalent of about 8.8-176.1 grams of pure hydrofluoric acid; at least 10 grams of a carbonic acid derivative; and at least about 1.5 grams of a monocarboxylic acid derivative containing alkali metal ions.
Coggins et al. U.S. Pat. No. 3,744,496 claims a milling composition for titanium and other refractory metals which comprises: about 210-630 grams of pure nitric acid; about 98-440 grams of pure phosphoric acid or its phosphate ion-producing equivalent; about 61-88 grams of pure hydrofluoric acid, or its fluoride-producing equivalent; and a carbonic acid derivative equivalent to about 15 grams or more of carbamide.
In Roni U.S Pat. No. 3,844,859, an improved method for milling titanium includes immersing metal in an aqueous fluid containing: a sufficient amount of hydrofluoric acid for effecting an etch rate of about 4-15 mils/side/minute; a sufficient amount of dodecylbenzene sulfonic acid and linear alkyl sulfonic acid for keeping the surface tension of this fluid between about 28-60 dynes/cm; and about 0.2-1.2 wt. % nitric acid. About 0.07-2.9 wt. % ammonium bifluoride may be added to this solution for reducing channeling and ridging in the fillet areas of a vertically-milled part.
Gumbelevicius U.S. Pat. No. 3,788,914 employs a titanium milling solution which contains, per liter of solution: about 126-682 grams of nitric acid; the equivalent of about 8.8-176.1 grams of pure hydrofluoric acid; and at least about 10 grams of a carbonic acid derivative selected from carbamide, urea nitrate, urea oxalate and semi-carbazide.
Kreml U.S. Pat. No. 3,666,580 discloses a milling solution comprising 2-10 vol. % hydrofluoric acid and 1-10 vol. % hydrochloric acid, with a remainder of water. This solution is maintained at a temperature between 65-140.degree. F. for the milling of titanium metal parts therein.
The milling composition of Snyder et al. U.S. Pat. No. 2,981,610 contains: about 0.1-4.7 molar nitrate; about 0.1-2.2 molar chloride; about 0.25-5.3 molar fluoride; at least about 0.22 normal acetate; and a hydrogen ion concentration of about 2.8-10.7 molar.
Current practices for chemically milling, etching and pickling titanium workpieces employ baths of hydrofluoric acid and nitric acid in various concentrations. Hydrofluoric acid poses risks to the health of its day-to-day handlers, however. Any process that employs HF poses another major risk in the event of an accidental release into the environment. Because of these concerns, hydrofluoric acid is being considered for greater Federal regulation. Nitric acids, on the other hand, release visible fumes of toxic NO.sub.x during standard milling operations. Emission source locations are also under increasing regulatory pressure to reduce or eliminate such emissions from the workplace. Although hydrofluosilicic acid (H.sub.2 SiF.sub.6) has been proposed as an HF substitute, this liquid is also hazardous and quite volatile.